chore: rebrand HexaUI / HexUI -> HexKit + README mark/gallery

[quanghexa94]

What

Pure prose rename across 56 files. Every product-facing reference to HexaUI or HexUI becomes HexKit; every internal identifier stays exactly as it was.

Scope (deliberately minimum)

The brand is the product surface — docs, docstrings, comments, alt text, error messages. Internal identifiers are not the brand and shouldn't change just because the brand did:

Stays as-is	Was	Why
Python module platform_backend	(unchanged)	Touching it means rewriting every from platform_backend … import + invalidating every dev .venv. Not worth doing for a brand pass.
Python module hexa_events	(unchanged)	Same logic; renaming requires updating every importer in proxy + agent-server + tests.
Python module agent_server	(unchanged)	Never named after the brand.
npm package agent-ui	(unchanged)	Already a neutral name.
Directory custom-UI/	(unchanged)	Cosmetic — separate cleanup PR.
Directory proxy-server/	(unchanged)	Already a neutral name.

The one non-prose change:

Changed	From	To
proxy-server/pyproject.toml	name = "hexaui-proxy"	name = "hexkit-proxy"

Caught by the same case rule as the rest. No external consumer pins this name yet, so the rename is free.

Restored URL

The README CI badge URL is now github.com/HexamindOrganisation/hexkit (matches the renamed GitHub repo).

Verification

make check is green:

• Ruff clean on both Python packages
• 90/90 proxy tests pass
• tsc --noEmit clean on both custom-UI and front-app

Diff shape

56 files, +124 / -124 — pure substitution. No identifiers added or removed; nothing that would trip a behavior test.

Follow-ups (not in this PR)

Discussed but explicitly deferred to keep this PR a clean rebrand:

• Rename Agent-Package/ workspace directory → hexkit/
• Rename custom-UI/ → agent-ui/ (matches its npm package)
• Decide what to do with packages/hexa-events/ (fold into proxy or publish)
• Eventually rename platform_backend → hexkit_server if internal identifier consistency matters

[quanghexa94]

Folded the icon + gallery work into this PR — not worth a separate one.

Two extra changes on top of the pure prose rename:

• Brand mark in the README header. The landing page's hexkit-mark.svg is now in assets/hexkit-mark.svg and displays above the title.
• New Components section between the agent table and the "two things you configure" section. 2x4 HTML grid of eight widgets (ai-response, ai-chat-input, metrics, table, tool-calls, markdown, form, button-group) — each card has the widget type, a one-liner, and a real ui.yaml snippet. The snippets use the canonical schema (the keys actually shipped in demo/agent-server/src/agent_server/ui/*.yaml), not the toy schema the landing page currently shows — that way a visitor following the README to the source examples doesn't hit a syntax mismatch.

Diff: +158 lines on top of the rebrand. make check is still green.

Single commit on top: docs: README mark + widget gallery section.

[quanghexa94]

Rebased onto main and cleaned up the items found in review.

1. Rebase resolved. Conflict was in demo/agent-server/src/agent_server/agents/devops_agent.py: main moved to_native_event out to google_adk.py while this branch was renaming HexaUI → HexKit. Took main's structural change (file no longer carries the function) plus the prose rename on top.
2. Agent table now includes ITSM and HR (both langchain/deepagents).
3. README repo layout: "6 sample agents" → "8 sample agents", names all eight.
4. QUICKSTART step 4: "six demo agents" → "eight demo agents", list now includes ITSM and HR.
5. Components section: was "5 more widgets ... placeholder" — but placeholder.tsx is a file in custom-UI/src/widgets/ that isn't in the registry, so it's not a user-facing widget. Now "4 more widgets" (dropdown, page-header, page-footer, spacer). Canonical count from custom-UI/src/registry/builtin.ts is 12, matching the repo-layout claim.

make check still green: 90/90 tests, ruff clean, both tsc projects.

Mergeable now. PR title left as-is since the bundled scope hasn't changed.